MO description

Section 2 22 Lewis structures orbital hybridization and molecular orbital descriptions of bonding are all used m organic chemistry Lewis structures are used the most MO descriptions the least All will be used m this text... 

It is worth noting that in the case of the reactions of ediylene and butadiene with the allyl cation, the MO description has provided a prediction that would not have been recognized by a pictorial application of valence bond terminology. Thus, we can write an apparently satisfactory description of both reactions. 

The examples that have been presented in this section illustrate the approach that is used to describe structure and reactivity effects within the framework of MO description of structure. In the chapters that follow, both valence bond theory and MO theory will be used in the discussion of structure and reactivity. Qualitative valence bond terminology is normally most straightforward for saturated systems. MO theory provides useful insights into conjugated systems and into effects that depend upon the symmetry of the molecules under discussion. 

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.)... 

The structure of ferrocene and an MO description of its bonding have already been given (p. 937). The rings are virtually eclipsed as they are in the analogous ruthenocene (light-yellow, mp 199°C) and osmocene (white, mp 229°C). 

On the other hand, this MO description matches a crystal field description of a Cu(II) ion insofar as the five highest occupied MO s are concerned four of these are doubly occupied and have a predominant copper 3 d character (65 % at least), whereas the fifth orbital contains the unpaired electron and still has 50% copper 3 d character. The very small MuUiken charge on the copper atom, which does not fit in the description of a Cu(II) ion, is caused by the partial occupation of the 4 p orbitals, which effect is neglected in the crystal field theory. 

The (triphos)Ni0 template reacts with ethyl cyanoformate to give (220).648 The MO description and reactivity of the CS2 complex (221) has been studied in some detail.649 Treatment with 02 gives the dithiocarbonate compound (221a),650 while bubbling of C02 through a solution of (221) yields the carbonato complex (221b).651 In both cases, one of the phosphine arms has been oxidized and is found dangling. 

Eqn. IV.3 is somewhat analogous to expressions arrived at earlier (say, Eqn. 11.12) the essential novelty is that Eqn. IV.3 emphasizes the valence situation of each individual Bl. In the standard application of MO descriptions to particular compounds there is implicit an adjustment procedure that is equivalent to an evaluation of an appropriate value for each parameter in Eqn. IV.3. The advantage of Eqn. IV.3 is thus that one is forced explicitly to perform each of these specializations. 

Now in addition to all these Si and Ti minima which have counterparts in the ground state surface, minima (or funnels) in Si and Ti can also be expected at biradicaloid geometries. These are geometries at which the molecule, in the MO description, has two approximately... 

We can also verify that the usual ECAO-MO description (3.2) and (3.4) leads to predicted hybridizations that are generally consistent with the donor-acceptor estimates (3.8). Suppose that each H atom is associated with a valence spin-orbital of hybridized form (3.6). According to Eq. (3.2), the optimal electronic energy of bond formation is obtained by choosing the hybridization parameter k to maximize the magnitude of the interaction element... 

Although the most naive form of valence-bond and Lewis-structure theory would not predict the paramagnetism of O2, the VB-like NBO donor-acceptor perspective allows us to develop an alternative localized picture of general wavefunctions, including those of MO type. Let us therefore seek to develop a general NBO-based configurational picture of homonuclear diatomics to complement the usual MO description. 

The NBO donor acceptor picture of hypervalency hyperbonds Let us re-examine the 3c/4e MO description from the localized NBO perspective. NBO analysis of the MO wavefunction (3.193) may lead to the Lewis structure A + B—C, with NBOs... 

Exercise For what ionicity tAB is the maximum MO bond order b wMO] obtained How does this maximum value compare with that of the Pimentel MO description ... 

In order to contrast the MO-type and VB-type descriptions, we may say that the former attempts to describe the MOs (or NOs) in terms of general mixings of AO basis functions (LCAO-MO description),... 

Problem Give a comparable sd"-based localized 2c/2e and 3c/4e description of octahedral FeH64 (which, like PtfLt2-, is an experimentally synthesized and characterized species) and compare it with the delocalized MO description. For the latter, assume Oh symmetry and employ the 15 AOs spanning the irreducible representations shown below ... 

The multiple bond to the metal in a rhenium(V) compound is best represented by the MO description of it being a triple bond, consistent with the symmetry of the px and p orbitals on the ligand see 1 (1). In keeping with that, these bonds are found to be relatively short and quite strong. The structure has also been presented as 1 a valence bond formulation that facilitates the counting of oxidation states but does not provide an accurate representation. 

The most widely used qualitative model for the explanation of the shapes of molecules is the Valence Shell Electron Pair Repulsion (VSEPR) model of Gillespie and Nyholm (25). The orbital correlation diagrams of Walsh (26) are also used for simple systems for which the qualitative form of the MOs may be deduced from symmetry considerations. Attempts have been made to prove that these two approaches are equivalent (27). But this is impossible since Walsh s Rules refer explicitly to (and only have meaning within) the MO model while the VSEPR method does not refer to (is not confined by) any explicitly-stated model of molecular electronic structure. Thus, any proof that the two approaches are equivalent can only prove, at best, that the two are equivalent at the MO level i.e. that Walsh s Rules are contained in the VSEPR model. Of course, the transformation to localised orbitals of an MO determinant provides a convenient picture of VSEPR rules but the VSEPR method itself depends not on the independent-particle model but on the possibility of separating the total electronic structure of a molecule into more or less autonomous electron pairs which interact as separate entities (28). The localised MO description is merely the simplest such separation the general case is our Eq. (6)... 

Scheme 1 Valence-bond- and MO-description of p-C-Si-a-bond hyperconjugation...

Annelation of additional aromatic units to the basic cis-1,2-diphenylethylene system exerts strong effects on its inherent reactivity. In the usual MO description these effects can be traced to the effect of the structure of the new skeleton on the highest occupied and lowest unoccupied orbitals at the atoms forming the new bond and therefore can be properly considered as topological effects. As such effects are quite numerous we shall limit ourseivs to only a few examples. Thus o-terphenyl 103) does not give any DHP under usual conditions ... 

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